1. Field of the Invention
This invention relates to a composition and process for removing moisture from a hydrogen halide.
2. Description of the Prior Art
At the present time, gaseous and liquid hydrogen halides are utilized as high purity anhydrous compositions. Anhydrous hydrogen halides are commonly used in the semiconductor industry such as for cleaning reactor tubes and susceptors and as an etchant for manufacturing microcircuits.
In such applications, highly efficient water vapor or liquid removal from hydrogen halides such as hydrogen chloride is required before its introduction to the end-use environment. Hydrogen chloride is normally a gas and sometimes is transported, under pressure, as a liquid. Hydrogen chloride becomes a liquid at 615 psig. Water-containing hydrogen chloride is highly corrosive in character, and thus will necessitate frequent replacement of piping, manifolds, valves, etc., with which it comes into contact. In cleaning susceptors, i.e., the support structures on which wafers are processed, the presence of water in the hydrogen chloride will result in the formation of new oxides on the susceptor, thus opposing the cleaning function which is sought to be carried out. In etching applications, water-containing hydrogen chloride is a source of undesirable moisture contamination in the semiconductor manufacturing environment, which may render the microcircuitry chip products made in such an environment deficient or even useless for their intended purpose.
Among the methods which have been utilized by the prior art for removing water from hydrogen chloride is the use of moisture-sorptive molecular sieves. The difficulty of employing such methods for production of high-purity hydrogen chloride is that hydrogen chloride is competitive with water for the absorption sites on the molecular sieves. As a result, it is not possible to obtain the necessary lower residual water values, on the order of 10 parts per million by volume concentration and less, in the effluent from the molecular sieve contacting step.
Hydrogen chloride has also been treated with sulfuric acid or phosphoric acid to produce dehydrated hydrogen chloride. Such dehydration methods, however, have the associated disadvantage that they add sulfur or phosphorous to the hydrogen chloride, and these added elements are highly undesirable contaminants in the aforementioned semiconductor manufacturing applications.
It has also been proposed to utilize magnesium chloride supported on alumina to effect removal of moisture from a hydrogen halide. It has been found that this purifying material is undesirable when contacted with high pressure hydrogen chloride such as in its liquid form since aluminum reacts with the hydrogen chloride to form aluminum trichloride particles which clog filters through which the hydrogen chloride is passed to effect its purification.
In addition, the formation of magnesium chloride on alumina involves multiple reaction steps wherein the alumina is first coated with a solution, e.g., 15% by weight of dibutylmagnesium in hexane solvent. The solvent is removed by evaporation while heating. The dibutylmagnesium is converted to magnesium hydride on alumina by heating to about 250° C. The magnesium hydride then is converted to magnesium chloride on alumina with concentrated hydrogen chloride. This composition then is used to remove moisture from hydrogen halides.
Hydrogen bromide is another example of a hydrogen halide which is required in essentially completely water-free condition in the semiconductor manufacturing field. Hydrogen bromide is used in the electronics industry as an etchant for wafers, and as a cleaning agent for susceptors. In these applications, the presence of water impurity in the hydrogen bromide will result in the same disadvantages noted hereinabove in connection with hydrogen chloride in similar applications. In addition, when hydrogen bromide is used as an etchant for wafers, hazing has been found to result when the hydrogen bromide contains even minute amounts of water vapor.
The art has attempted to achieve removal of water from hydrogen bromide by the use of phosphoric acid as a drier. This method, while generally useful to remove the water contaminant, nonetheless has the attendant disadvantage that it adds phosphorous to the hydrogen bromide, which as indicated above in connection with hydrogen chloride, is a significant contaminant in the semiconductor manufacturing process.
Accordingly, it would be desirable to provide a composition and process for removing moisture from hydrogen halides and which does not produce a contaminating by-product such as particles. In addition, it would be desirable to provide such a composition and process which has a high capacity for removing moisture from hydrogen halides either in gaseous or liquid form. Furthermore, it would be desirable to provide such a composition which can be formed from a simplified process as compared to presently available processes for forming analogous compositions.